The present invention relates to charging systems for electric vehicles, and more particularly to a power-factor-corrected, single-stage inductive charger for use in inductive charging of electric vehicle batteries.
Electric vehicles are gaining popularity due to increased emissions requirements. The electric vehicles require a highly efficient, reliable and safe electric drive train to compete with vehicles powered by internal combustion (IC) engines. The electric vehicles rely upon batteries that must be charged periodically. Inductive charging systems or converters are typically used to charge the batteries.
Conventional inductive charging systems include two main components. The first component includes an inductive charger that is located separately from the electric vehicle. The inductive charger conditions and converts low-frequency AC supply power to high-frequency AC power. An inductive coupler (or plug) connects the inductive charger to the electric vehicle.
The second component includes an on-vehicle inductive inlet (or socket) that mates with the inductive coupler of the inductive charger. The high-frequency AC power that is provided by the inductive charger is transformer-coupled to the electric vehicle via the inductive inlet. The high-frequency AC power is rectified and filtered to generate a DC current that charges the batteries.
The current electric vehicle inductive chargers that are manufactured by the assignee of the present invention are known as Standard Charge Modules and Convenience Charge Modules. These inductive chargers have two series power stages that process the power from the utility line to the inductive coupler. The first stage, which is typically a boost-type converter, corrects the power factor of the current that is drawn from the rectified low-frequency AC power line. The power factor correction maximizes the available power and minimizes the AC line current and voltage distortion. The first stage additionally converts the rectified utility low-frequency AC power to high-voltage DC by filtering using large electrolytic capacitors.
The second power processing stage has two functions: (1) controlling the output power to the battery and (2) conditioning the high-frequency AC voltage and current for input to the inductive cable and coupler. The second stage includes a resonant inverter with MOSFET switches and a series tank circuit that includes an inductor and a capacitor. The resonant inverter chops the high voltage DC that is produced by the first stage into high-frequency AC. The high-frequency AC is filtered by the series tank circuit and fed into a cable that is connected to a winding of the inductive coupler. A parallel or resonant tank shunts energy away from the battery as the frequency is increased.
The resonant inverter operates at a frequency above the natural frequency of the series tank circuit to enable soft switching of the inverter MOSFETs. The super-resonant operation also provides highly efficient power transfer. The power transferred from the utility to the battery is regulated by controlling the operating frequency of the resonant inverter. Decreasing the operating frequency increases current to the battery. Increasing the operating frequency decreases current to the battery.
The inductive charging for electric vehicles is standardized using the Society of Automotive Engineers Inductive Charge Coupling Recommended Practice, SAE J-1773. SAE J-1773 defines a common electric vehicle conductive charging system and architecture and the functional requirements of the vehicle inlet and mating connector. The inductive charging vehicle inlet that is defined by SAE J-1773 contains two passive elements; the transformer magnetizing inductance; and a discrete capacitance connected in parallel with the transformer secondary.
An inductive charger according to the present invention is capable of charging using both single-phase and three-phase power sources. A rectifier is capable of being connected to both single- and three-phase power sources. A capacitor is connected across an output of the rectifier. An inverter is connected to the rectifier and the capacitor and includes a plurality of switching circuits. A series resonant tank circuit is connected to an output of the inverter. A charge coupler and an inductive inlet include a transformer and a parallel resonant tank circuit for coupling energy to a load. A controller is connected to the rectifier and the inverter. The controller generates drive signals for controlling the switching circuits, operates the inductive charger in a super-resonant mode and regulates output power to the battery by modulating an operating frequency and output current of the inductive charger around an input frequency and an input current.
In other features of the invention, the parallel tank circuit includes a first inductor connected in parallel to a first capacitor. The series tank circuit includes a second inductor connected in series to a second capacitor. The load is a battery of an electric vehicle.
In still other features, the inverter is a full-bridge or half-bridge inverter. The full-bridge inverter includes four switching circuits each with a controlled switch, an anti-parallel diode, and a snubber capacitor. When the power supply is a single-phase power supply, the inductive charger is capable of providing a power factor of approximately 0.99. When the power supply is a three-phase power supply, the inductive charger is capable of providing a power factor of approximately 0.91.
In still other features, the rectifier includes first, second and third legs. Each of the first, second and third legs include first and second diodes. Alternately, the first and second legs include first and second diodes and the third leg includes first and second controlled switches such as silicon controlled rectifiers.
The controller includes an amplifier that generates a current reference signal from the input current. An attenuator produces a voltage reference signal from the input voltage. A multiplier generates a product signal based on the voltage reference signal and a power command signal. A differential circuit generates an error signal and has an inverting input connected to the current reference signal and a non-inverting input connected to the product signal. A compensator generates a compensated signal from the error signal so that the error signal is minimized, thus correcting the power factor by making the input current and input voltage have the same shape. A gate driver produces drive signals for the switching circuits of the inverter.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.